Fissionable nuclear fuel for nuclear reactors typically comprises one of two principal chemical forms. One type consists of ceramic or non-metallic oxides of fissionable and/or fertile element(s) such as uranium, plutonium or thorium, and mixtures thereof. This category of ceramic or oxide fuels are disclosed, for example, in U.S. Letters Pat. Nos. 4,200,492, issued Apr. 29, 1980, and 4,372,817, issued Feb. 8, 1983. Among certain distinctive aspects and advantages of ceramic fuels is their relatively high melting temperatures and low level of reactivity with other materials even at high temperatures and pressures encountered in nuclear reactors. Ceramic fuels commonly comprising uranium dioxide are most frequently used in contemporary commercial electrical power generating, water cooled and moderated nuclear reactors. The very high melting characteristics and low level of reactivity of these oxide fuels enables the reactor to be operated at higher temperature and in turn pressure conditions for increased efficiency and economy. Typical oxide forms of fuel such as uranium dioxide have melting temperatures of approximately 5000 degrees F. (2760 degrees C).
Ceramic or non-metallic fuels however suffer from low thermal conductivity and lower fissionable atoms per unit volume, among other shortcomings.
The other principal type of nuclear reactor fuel comprises fissionable elements such as uranium, plutonium and thorium, and mixtures thereof, in metallic, non-oxide form. Specifically this category comprises uranium, plutonium, etc. metal and mixtures of such metals, namely alloys of such metals. Fissionable fuel materials in metal form have a long history in the nuclear reactor field, but due to significant shortcomings have currently been used primarily in the so-called breeder type of reactors utilizing liquid metal coolants.
The primary handicaps in the use of metallic fissionable material as fuel are their relatively low melting temperatures and accompanying loss of structural characteristics such as tensile strength at moderately low temperatures, and high degree of reactively with other elements including susceptibility to corrosion. For example uranium, the most prevalent fissionable fuel material, as a metal melts at only 2070 degrees F. (1132 degrees C.), and an alloyed metal fuel containing uranium and plutonium normally have even lower melting temperatures, such as about 1130 degrees F (610 degrees C.) for an 88 weight percent uranium and 12 weight percent plutonium alloy.
Fissionable fuels in metal form, on the other hand, provide excellent thermal conductivity for highly efficient heat transfer, and maximum concentration of fissionable atoms per unit volume, among other attributes. Thus, more power can be produced per unit size with metallic fuel, and more efficiently conveyed to the heat carrying coolant.
Alloying mixtures of uranium metal and/or plutonium metal have been proposed and used to enhance metal fuels and overcome such shortcomings. For example, small amounts of alloying metals such as molybdenum, niobium, titanium, zirconium, or chromium, have been used to stabilize phase structures and in turn the properties attributable thereto in metallic fuels.
For instance, the article "Properties Of Uranium-Plutonium-Base Metallic Alloys" by R. J. Dunworth et. al., Argonne National Laboratory, Annual Progress Report For 1965, ANL-7155, 1965, pages 14-25, discloses the alloying of metal fuel with zirconium or titanium to increase the melting temperature of uranium-plutonium fuel. Such alloyed fuel compositions, specifically those comprising a major portion of uranium metal with minor portions of plutonium and zirconium metals have been the subject of extensive consideration as evidenced in an article entitled "Performance Of Advanced U-Pu-Zr Alloy Fuel Elements Under Fast-Reactor Conditions" by W. N. Beck et. al., Argonne National Laboratory, Trans. Ans., 10, 1967, page 106 and 107. Zirconium is also included as an alloying component in such metallic fuels to provide an elevated solidus temperature for the fuel and to enhance its chemical compatibility with stainless steel which is commonly employed in fuel containers for service in liquid metal cooled nuclear reactors.
However, subsequent studies have identified additional problems which are not adequately resolved by alloying mixtures of such conventional fissionable metal fuels with non-fuel elements such as zirconium metal. It has been found that evidently due to inherent reactor conditions of intense radiation and high temperatures, metal alloy fuels, which initially are provided in a substantially uniform mixture of the alloyed ingredients, become chemically redistributed into nonuniform mixtures of the alloyed components. This restructuring phenomena in the metal alloyed fuel has a pronounced effect upon its properties and their distribution throughout the mass of the fuel body.
One significant aspect of this chemical redistribution of the alloyed ingredients such as zirconium, is the inward migration of the zirconium metal to the inner or central area of the body of fuel with an accompanying increase in the solidus temperature of the inner or central area and correspondingly a reduced solidus temperature in the outer or peripheral area of the unit. Thus, &he melting conditions of the remaining alloying ingredients, such as uranium and/or plutonium, in the peripheral area of the fuel body are lowered, and the effect of the added zirconium to avoid low melting phase formation is reduced or negated. A lower melting condition of the surface portions of fuel units increase the potential for chemical interaction with adjoining materials.
This redistribution phenomenon in metal alloyed fuel whereby zirconium as an ingredient migrates inward away from the peripheral area of the body of fuel results in the remaining alloy ingredients forming lower melting alloys or eutectic compositions. Moreover, components remaining in the peripheral area of the fuel such as plutonium and fission produced cerium, deprived of zirconium, result in low melting ingredients which can attack the stainless steel of the fuel container, and/or react with components of the stainless steel alloy such as iron and thereby diminish the integrity of the container. An interreaction between fuel components and the stainless steel container housing the fuel, or its ingredients such as iron, will detract from the structural strength of, the containers relatively thin walls, due to reduced thickness, altered composition or resulting permeability.
For instance a paper entitled "Chemical Interaction Of Metallic Fuel With Austenitic And Ferritic Stainless Cladding", by G. L. Hofman et al, Argonne National Laboratory, Tucson Conference, September 1986, discusses an interdiffusion phenomenon between metallic fuel comprising uranium/plutonium/zirconium and components of ferritic stainless steel fuel containers. It is noted that the phenomenon could have an adverse effect upon the performance of such a fuel composition housed within the conventional stainless steel containers. The potential of this phenomena comprises formation of strength reducing diffusion zones within stainless steel, intergranular penetration of fuel ingredients into stainless steel, and the formation of eutectic areas with low melting levels below that of operating temperatures.
The nonuniform melting conditions of metal alloy fuels in reactor service and the potential effect is a subject of an article "Postirradiation Examination Of U-Pu-Zr Fuel Elements Irradiated In EBR-11 To 4.5 Atomic Percent Burnup", by W. F. Murphy, et. al., Argonne National Laboratory, ANL-7602, November 1969. This article additionally discusses the extensive physical changes which occur in metal alloy fuel during fission such as its extensive deformation including expansion or swelling of up to about 30 percent by volume, including thermal expansion and expansion due to internal generation of fission produced gases.
Due to this extensive degree of swelling of metallic fuel during fission in reactor service, the body of fuel material, typically in the form of long thin rods and sometimes referred to as slugs, is provided with a cross-sectional dimension substantially less than the internal cross-sectional area of the container housing the fuel to accommodate the increased volume of the resulting irradiation gas bloated body of fuel.
Additionally, the potential for causing a failure of stainless steel containers housing metal alloy fuel is reviewed in a brief article entitled "Metallic Fuel Cladding Eutectic Formations During Postirradiation Heating" by B. R. Seidel, Argonne National Laboratory, Trans. Ans. 34, June 1980, pages 210 and 211.